1. Field of the Invention
The present invention relates to an image forming apparatus capable of changing a color mode to a subtractive color mode and vice versa for use.
2. Description of the Related Art
Earlier, an image forming apparatus capable of changing a color mode to a subtractive color mode and vice versa for use on the occasion of transferring toner images from a plurality of image bearing bodies to an endless belt-like intermediate transfer body or to a transfer material carried on the endless belt-like intermediate transfer body together with it has had the following two problems.
a) Peeling discharges are generated when the endless belt-like intermediate transfer body or the transfer material is separated from the image bearing bodies after transfers owing to a change of a resistance value of the endless belt-like intermediate transfer body or the transfer material, or owing to a rise of an electric potential of toner layers produced by superposing toner images colored in a plurality of colors.
b) Toners transferred onto the endless belt-like intermediate transfer body or the transfer material are changed in their charges and polarities at the time of the transfer of the next color or the color after the next, and are re-transferred onto the image bearing bodies.
As measures for solving the problems, the following means have been tried generally: decreasing a toner mass of deposit, decreasing a toner quantity of charging, decreasing a resistivity of the endless belt-like intermediate transfer body, selecting a transfer output in which the above-mentioned problems are difficult to occur, or the like. There was also a trail of combining the several measures for preventing the occurrences of the phenomena described above in the case of superposing a plurality of color toners, ordinarily two or more color toners.
In addition, there is also an image forming apparatus disclosed in, for example, Japanese Patent Application Publication (Unexamined) No. Tokukaihei-11-249371. The image forming apparatus is a color image forming apparatus using four color toners. When only a black toner is used in case of forming a monochrome image by this image forming apparatus, the other three color image bearing bodies are not made to contact with an endless belt-like intermediate transfer body or a transfer material carried together with it. In this case, only the transfer of the black toner should be considered. However, because transfer outputs in this case are set to be the same as those in case of transferring the four color toners, the output level of the black toner remains small. Consequently, such the image forming apparatus has a defect of resulting in an insufficiently transferred image in a monochrome mode.
Moreover, in the subtractive color mode using two color toners, normally, four image bearing bodies ordinarily contact with the endless belt-like intermediate transfer body, and in such a state, only two color toners are used. Consequently, transfers from the image bearing bodies of the residual two colors are unnecessary. However, in this case, the following problem is produced. That is, when a transferring image (a toner image) on which the above-mentioned two color toners have been transferred passes transfer positions of the image bearing bodies of the residual two color toners, the toners on the endless belt-like intermediate transfer bodies are re-transferred on the residual two color toner image bearing bodies.
For example, a technique disclosed in Japanese Patent Application Publication (Unexamined) No. Tokukai-2000-98758 copes with such problems by arranging a transfer section to each of the four image bearing bodies through the endless belt-like intermediate transfer body, and by setting transfer outputs of two transfer sections at two end positions on the uppermost stream and the lowermost stream to be larger than transfer outputs of two transfer sections at two positions in the central part. Moreover, because the transfer outputs of the transfer sections at the two end positions are set to be larger in a plural color mode than those in a monochrome mode, re-transferring becomes inevitable and good results are not necessarily obtained.